console.log(typeof a);    // undefined
console.log(typeof 1);   // number
console.log(typeof 'zhufeng'); // string
console.log(typeof true);  // boolean
console.log(typeof Symbol('a'));  // symbol
console.log(typeof function () { });  //function
console.log(typeof [1, 2, 3]);  //object
console.log(typeof { name: 'zhufeng' });  //object
console.log(typeof null);  //object
console.log(typeof new Number(1));  //object